This invention relates to an information recording medium and to a method of manufacturing a resinous substrate to be employed for the information recording medium. In particular, this invention relates to a surface recording/reproducing type information recording medium and to a method of manufacturing a resinous substrate to be employed for such an information recording medium.
An ordinary optical disk such as CD, CD-ROM, etc. is constructed such that emboss pits are formed in conformity with the recorded data on one of the surfaces of a transparent substrate having a thickness of 1.2 mm, the emboss pits being covered thereon by a reflective film made of Al for example. The information recorded in the CD constructed in this manner can be reproduced by irradiating a converging beam onto the emboss pits from a surface of the transparent substrate which is opposite to the other surface where the reflective film is formed.
On the other hand, an optical disk such as DVD, DVD-ROM where the recording density is highly enhanced is constructed such that finer emboss pits than those of the CD are formed on one of the surfaces of a transparent substrate having a thickness of 0.6 mm, the emboss pits being also covered thereon by a reflective film made of Al for example. The information recorded on the recording surface of the disk constructed in this manner can be reproduced in the same manner as that of the CD, i.e. by irradiating a converging beam onto the emboss pits from a surface of the transparent substrate which is opposite to the other surface where the reflective film is formed.
As for the material for the substrate having a thickness of 0.6 mm, PC (polycarbonate) which is a transparent resin is generally employed. This PC substrate having a thickness of 0.6 mm however is not sufficient in mechanical property, resulting in the warping of the substrate as it is employed singly. Therefore, in order to prevent the substrate from being warped, a couple of PC substrates each having a thickness of 0.6 mm are superimposed each other with the recording surface being directed inside, thus forming a disk having a total thickness of 1.2 mm, thereby ensuring the mechanical property thereof.
The reason for setting the substrate of DVD to 0.6 mm is to secure the tilt margin of the disk. When the density of track pitch or pit is increased, the margin of the inclination or so-called tilt of the disk is caused to decrease. Although it may be possible to secure the tilt margin by decreasing the thickness of the substrate from 1.2 mm to 0.6 mm, it will inevitably result in a deterioration of the mechanical property thereof.
Under the circumstances, there has been proposed, with a view to secure the mechanical strength of the disk while decreasing the thickness of the substrate, an idea of thickening the central portion of the disk thereby to ensure the mechanical strength thereof (Japanese Patent Unexamined Publication H9-204686). However, it is required, for ensuring a sufficient mechanical strength, to make the thickness of the signal recording region of the substrate at least 0.6 mm. Further, there is also reported an idea of making the thickness of the substrate to range from 0.1 mm to 0.6 mm (Japanese Patent Unexamined Publication H9-204688). However, the thickness of a protective substrate for sustaining the recording film as well as the film thickness of the reflective film are not referred to in the idea, thus making it difficult to practice in the actual application thereof.
U.S. Pat. No. 5,757,733 teaches an information recording medium comprising a covering layer formed on the light beam incident side, and a flat substrate sustaining a recording film. However, this covering layer is simply referred to as having a thickness of 0.6 to 1.0 mm.
For the purpose of further increasing the capacity of the disk by compacting the recording density, it is more effective, in view of assuring the tilt margin, to make the thickness of the substrate as thin as possible. However, when the thickness of the substrate becomes less than 0.6 mm, it becomes difficult to secure the mechanical strength thereof even if a couple of substrates are superimposed each other.
Moreover, this superimposition of a couple of substrates is accompanied with the problems that it not only requires the employment of an adhesive but also makes the manufacturing process thereof more complicated.